The sharemarket is telling BHP Billiton and Rio Tinto, in that subtle way it has of slashing prices, to stop digging and pay bigger dividends.

It strikes me as being pessimistic about everything at the moment, though one of us is going to be wrong, possibly depending on who changes their mind first.

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But there's no disputing a big loser will be Canberra's long-drawn-out iron ore and coalmining super tax.

It could be the first tax ever not to raise anything, which is not such a bad precedent come to think of it but no surprise, considering it was the big miners who redrafted it. Not a peep from them since the legislation was passed, either. Their only worry is other countries may get ideas and bring in a resources tax as well, only do it properly.

But back to the boom. Those megabillion-dollar investments mean big diversified miners such as BHP Billiton and Rio Tinto can produce more later and in this game more is, well, more. It's all about supply and demand, except sort of back-to-front. Even when prices drop, they can step up output and let economies of scale get them back to where they started. Not many businesses can do that.

Nirvana, mind you, is producing more as the price rises but those days were never going to last because others get in on the act, not to mention that most of China's steel makers who buy our iron ore and coal make a loss, which can't go on either.

So commodity prices would have to plunge for their earnings to fall as much as their share prices have in the past year, which is about 30 per cent. Far from signs of that, if anything earnings are being boosted by the weaker dollar.

But the market, spooked both by China's growth slowing and shift in emphasis from steel-intensive infrastructure to namby-pamby consumer goods, has this funny need for perpetual motion. If something isn't growing, it's dead.

Yet so big has its economy grown that even demand staying put would keep our miners going for years.

The only worry is competition from other suppliers and even then, as prices drop high-cost mines become uneconomic.

Besides, all the big miners need do is dig or drill and the exchange rate will take care of the rest.

Commodity prices drop; so will the dollar. They rise and it goes up, too, and that didn't hurt them, did it?